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Old 19th May 2009, 12:52 AM   #1
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Default Blown LM3875


All things where nice and smooth, sounding great even. But then tonight one of the channels on my Audio Sector kit blew up. The chip is cracked and has the wonderful smell we all know.
Assuming that my rails are nice and smooth (this will be the first thing I check) and the rectifier is in tact what would be the next step in fixing this?
I do not have any way to measure the caps but I can check the rest with my simple multimeter. I plan on picking up a new LM3875 tomorrow along with a couple new resistors. I may even build up a new rectifier board.

Would a short or cold joint cause this to happen? If so where would be the best place to look? I am thinking that it could have been the tiny RF killer cap I put between pin 7-8 on the chip. It is pretty tight, there might have been something there that I did not see.

It just seems odd that all was well and then it just blew. No physical movement. Nothing changed. In fact there was not even a speaker connected to that channel. The amp was running but at very low levels. No where near enough to get anything anything hot. The transformer is bran new and it is plugged into a regulated UPS.

Thanks for any advice.
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Old 19th May 2009, 03:54 AM   #2
wwenze is offline wwenze  Singapore
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Maybe the screw connecting it to the heatsink is too tight? Just a long guess.
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Old 19th May 2009, 04:57 AM   #3
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Ya I hear ya. I am at a bit of a loss. It could have been something that simple. The board has taken a pretty bad beating now that I have taken the chip off. I am not sure if I trust the board I have spent the last hour checking traces. I have to say that by doing this I am gaining a much better understanding of what is going on with this thing. So I guess there is a bright side to all this.
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Old 19th May 2009, 05:08 AM   #4
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Did it get HOT? It could have been oscillating.
Does it have a Zobel network on the output?
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Old 19th May 2009, 05:39 AM   #5
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No not that I could tell. It happened while I was doing some soldering and I thought the evil smell was from hitting some plastic. But then I quickly realized that it was something much worse. I quickly powered down the amp and unplugged it to give it a good sniff and there it was a nasty chip. It sat there blown for about three or so min. The heat sink was a little warm.

There is no Zobel network on the outs.

Could previous high temps have damaged the chip slightly then causing it to fail later? Or would it simply die at the time of overheating?
I also touched the transformer to see if it had gotten hot. It was ever so slightly warm. I have not felt the transformer before to check for heat so I am not sure what is "normal" for it.

I was tried to check the voltage coming off the rectifier but it looks like my multimeter is on the fritz. My numbers where all over the place. It was the same on the working channel so I am pointing to the meter. I am trying to get one tomorrow along with my new parts. Also now that I am aware of what pins are in use I think I may just break off the ones that are in the way to ensure that there are no shorts.
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Old 19th May 2009, 02:18 PM   #6
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So you were soldering while the amp was on ?
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Old 19th May 2009, 04:30 PM   #7
wwenze is offline wwenze  Singapore
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LM3875 has built-in overheat protection and can withstand up to 150 degrees during operation, IIRC.

Were you soldering while the amp was on?
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Old 19th May 2009, 04:35 PM   #8
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Quote:
Originally posted by MJL21193
Did it get HOT?
Of course it did.

A short between pins 7 and 8 will do no harm to the IC. That would mean 0 difference from one input to the other, hence 0 output.

The only shorts that could damage the IC can happen at the output. Either from output to ground, when a high signal is present or from output to either of the rails, when a small or no signal is present.

Oscillations could also heat the chip up, and they would happen exactly, when no speaker is connected and no Zobel is present. Then there will be no impedance to dampen them. So either add a Zobel network or make sure that a speaker is always connected.
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Old 19th May 2009, 05:21 PM   #9
AndrewT is offline AndrewT  Scotland
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Plug in your bulb tester.
Check the output of the transformer.
Then add on the rectifier and smoothing. Check the output again.
Add the Zobel to the output or even better add a Thiele Network to the output.
Add on a good channel of the amplifier. Check the voltages on the transformer, at the smoothing caps and at the amplifier output.
Check the output offset of the amplifier.
Remove the good channel and add on your repaired channel. Go through those checks again.

Has the bulb stayed off through all this testing?
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Old 19th May 2009, 05:33 PM   #10
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Quote:
Originally posted by pacificblue
Oscillations could also heat the chip up, and they would happen exactly, when no speaker is connected and no Zobel is present. Then there will be no impedance to dampen them. So either add a Zobel network or make sure that a speaker is always connected.
Properly operating amp does not require speakers, or Zobel connected.

I often had them run for days without anything connected and that does not affect their operation.
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